GitLab

Update memory histograms description and expiry dates

The histograms were updated to record based on random sampling. Update
the description to specify this. Also update the expiry dates of
memory histograms.

BUG=1117570

Change-Id: Ia03d9f6b598bc832aa7411fbc27e427c1436a968
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2381065
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809099}